1. Field of the Invention
This invention relates to a tracking error control apparatus and method for controlling the tracking error of a light beam in an optical disk apparatus for recording and reproducing the information by the land track/groove track recording method on an optical disk such as the DVD-RAM and the like.
2. Description of the Related Art
Various high-density optical disk formats have been proposed in accordance with the increased demand for a larger capacity in recent years. The land track/groove track format is an example (DVD-RAM disk, etc.) among them. This format includes a convex land track and a concave groove track where polarities of tracking control are inverted alternately with each other in radial direction of the disk. These tracks are spirally formed on the disk, and reproduction is carried out after information is recorded in these tracks.
A header area is formed at the head of each sector of the land and groove tracks. The header area is the area where the address information called CAPA (complementary allocated pit address) is formed. The header area consists of pre-pits so that the address information can be extracted irrespective of whether the read optical head is located on the groove track or the land track. The header area includes a CAPA1 area and a CAPA2 area.
In the optical recording and reproducing apparatus for recording and reproducing information on an optical disk, the focus control operation for keeping the light beam always in a predetermined converged state on a material film and the tracking control operation for controlling the light beam to always scan a predetermined track correctly, are performed. The shift amount of the track for tracking control operation is detected based on the light reflected from the disk like the signal.
Next, the tracking detection method generally called the push-pull method is explained. The signal used for tracking error control operation is hereinafter referred to as the TE signal. The push-pull method is also called the far field method. In this method, the light reflected and diffracted in the guide groove on the disk is received by optical detectors which is split into two parts and arranged symmetrically about the track center, and the output difference between the optical detectors is detected as a TE signal. As long as the center of the light beam spot and the center of the land or groove track coincide with each other, the outputs of the two optical detectors have a symmetric reflection and diffraction distribution. Otherwise, the light strength is varied between the outputs of the two optical detectors. Therefore, in the state where the beam spot is located at the center of the land track or the groove track, the TE signal becomes zero level. As the tracking control operation utilizes this characteristic of the TE signal, the tracking error control operation is performed by moving the beam spot on the disk in the orthogonal direction to the track in accordance with the TE signal. Specifically, the light beam spot (focusing lens) is moved in the direction perpendicular to the track by a tracking actuator in accordance with the TE signal.
In the TE control configuration (control configuration according to the push-pull method) described above, the TE control accuracy is reduced in cases where a radial tilt has been generated in the disk, as described in U.S. Pat. No. 6,459,664, US 2002/0051411, US 2004/0105362, U.S. Pat. No. 5,444,682 and US 2003/0133387. Consequentially, the conventional configuration has been proposed in which a second TE signal is generated based on the light beam reflected when passing through the CAPA1 area and the light beam reflected when passing through the CAPA2 area, and the TE signal described above (hereinafter referred to as the first TE signal) is corrected based on the second TE signal thus generated. Thereby, it is possible to further enhance the control accuracy of a positional shift between the light beam and the track.
In the optical disk having the land track and the groove track, different DC offsets are generated to the first TE signal in the land track and the groove track due to the radial tilt, etc. Therefore, direction of track deviance is opposite in the groove track G and the land track L, there are cases that the relative magnitudes of the amplitude are reversed between the second TE signal obtained from the CAPA1 area and the second TE signal obtained from the CAPA2 area. For this reason, some length of time is required before the second TE signal is stabilized (control is converged) at the track switching point (the timing at which the polarity of the second TE signal is switched).
The frequency band in which the first TE signal can be corrected based on the second TE signal generated in the header area formed discretely on the land track or the groove track, however, is set at a lower level comparing to the tracking control operation in view of the object to be corrected. Due to such reason, in the tracking error control operation for correcting the first TE signal based on the second TE signal, therefore, the time required to stabilize the second TE signal at the track switching point (the timing at which the polarity of the TE signal is switched) is lengthened as compared with the corresponding time for the ordinary tracking error control operation. As a result, the track deviance at the track switching point cannot be instantaneously corrected.
The currently available optical disk apparatus requires a higher recording and reproducing rate, and the aforementioned problem on a correcting rate of the tracking deviance is critical to meet the demand for a higher recording and reproducing rate.